In effect, currently, the manufacturers of cell phones who have ventured into the touchscreen mobile sector are seeking to be distinguished by incorporating a haptic interface. Thus, when a user slides his or her finger over an icon situated on the screen, he or she can feel a slight vibration originating from the mobile, a sensed vibration returning to him or her the impression of pressure on a button.
In this context, the applicants have developed a solution using thin piezoelectric layers as actuator and have demonstrated the possibility of obtaining a haptic field by using a piezoelectric material such as PZT (lead zirconate titanate) which makes it possible to actuate a plate mode such as the antisym metrical Lamb or Rayleigh wave propagation mode. This vibration mode induces an air blade called “squeeze film” effect between the finger and the plate, which provokes a modification of the friction coefficient between the plate and the finger. A vibration of the wave created in the plate induces a controlled variation of the friction coefficient between the finger and the plate.
Generally, the vibration modes are well controlled in the context of a rectangular plate and are disrupted when the geometry used is unconventional and has, for example, rounded corners. In this case, a solution has to be found to the problem of wave propagation disturbance in this type of geometry.
A large number of references report on the study of these vibration modes on various structures and the propagation of the Lamb waves has been the subject of numerous studies, but generally intended for non-destructive inspection of structures.
There are thus many references relating to the study of the propagation of Lamb waves in rectangular plates, and articles that can notably be cited are those of: P. Sergeant, F. Giraud, B. Lemaire-Semail, “Geometrical optimization of an ultrasonic tactile plate”, Sensors and Actuators A 161, pp. 91-100, 2010, and M. Biet, F. Giraud, B. Lemaire-Semail, “Squeeze film effect for the design of an ultrasonic tactile plate”, IEEE transactions on ultrasonics, Ferroelectrics and Frequency control, vol. 54, no 12, December 2007, pp. 2678-2688, in relation to haptic systems.
Other articles study the propagation of the Lamb waves in rectangular plates for the purposes of non-destructive inspection of structures: Y. Gomez-Ullate and al, “Lamb waves generation in plates using glued piezoceramics”, bulletin from the Spanish company Ceramica y Vidrio, vol 45, 2006, pp. 188-191.
Studies have already been made on non-rectangular plates, or plates having particular features at their ends. In effect, there are articles reporting on the study of beveled plates, notably in aerospace, or the bevel at the end of the plate is induced by the techniques of assembly of the various parts. This type of study can be found in the article by M R. Mofakhami and C. Boller, “Lamb wave interaction with non-symmetric features at structural boundaries”, GAMM 2008, N. Wilkie-Chancellier. In these publications, the aim is to study the reflection of the Lamb wave on this type of particular feature in order to conduct a non-destructive inspection. What is not reported or studied is how to overcome the particular feature at the end of the plate to retain the Lamb wave.
Many references report the study of the propagation of Lamb waves in rectangular plates with defects such as discontinuities: F. Benmeddour and E. Moulin, “Generation of a selected Lamb mode by piezoceramic transducers: Application to nondestructive testing of Aeronautical structures”, in Piezoelectric materials and devices—Practice and applications, ISBN 978-953-51, published Feb. 27, 2013], circular or rectangular holes, through or blind: O. Diligent, “Interaction between fundamental Lamb modes and defects in plates”, Thesis of the University of London, 2003, M V. Predoi, A. Negrea, “Ultrasonic guided waves sensitivity to flaws near plate edge”, UPC. Sci. Bull., Series D, Vol 72, Iss. 2, 2010. Here again, the aim is to study systems for non-destructive testing of structures. The Lamb wave is used to reveal the defect and overcoming it is not studied.
There are also references studying the vibrations of the rectangular plates with rounded corners (super elliptical plates): S. Ceribasi, G. Altay, “free vibration of super elliptical plates with constant and variable thickness by Ritz method”, Journal of Sound and Vibration 319, 2009, pp. 668-680. However, none of these references proposes any solution making it possible to retain, in these plates with rounded corners, a mode such as that which can be obtained in a rectangular plate, for example by modifying the limit conditions.
The circular plates are also covered in the article by M. Destrade, Y B. Fu, “A wave near the edge of a circular disk”, The open acoustic journal, 2008, pp. 15-18 which studies, for example, a surface wave in a disk, a study which does not impinge on the present invention.
To sum up, the above-mentioned references are essentially geared to the non-destructive testing of structures and in which the vibration mode is used to reveal defects. These references do not present any solutions making it possible to overcome the defect present to obtain a wave of desired form.
To address this problem of presence of a defect, the applicants considered it more relevant to move away from the issue (plate, Lamb wave) to turn toward the issue of acoustic resonators, for which the aim is precisely to retain the vibration mode with the best possible quality factor.
In the case of surface acoustic wave resonators, SAWs, a system of interdigital combs is used to generate a surface wave, propagated in the plane of the resonator. To optimize the quality factor of the device and avoid a loss of energy with a dispersion of the wave, reflectors are arranged on either side of the interdigital comb system, as shown in the article by M F. Hribsek, D V. Tosic, M R. Radosavljevic, “Surface Acoustic Wave sensors in mechanical engineering”, FME transactions, 38, 2010, pp. 11-18.